/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * A counting semaphore.  Conceptually, a semaphore maintains a set of
 * permits.  Each {@link #acquire} blocks if necessary until a permit is
 * available, and then takes it.  Each {@link #release} adds a permit,
 * potentially releasing a blocking acquirer.
 * However, no actual permit objects are used; the {@code Semaphore} just
 * keeps a count of the number available and acts accordingly.
 *
 * <p>Semaphores are often used to restrict the number of threads than can
 * access some (physical or logical) resource. For example, here is
 * a class that uses a semaphore to control access to a pool of items:
 * <pre> {@code
 * class Pool {
 *   private static final int MAX_AVAILABLE = 100;
 *   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
 *
 *   public Object getItem() throws InterruptedException {
 *     available.acquire();
 *     return getNextAvailableItem();
 *   }
 *
 *   public void putItem(Object x) {
 *     if (markAsUnused(x))
 *       available.release();
 *   }
 *
 *   // Not a particularly efficient data structure; just for demo
 *
 *   protected Object[] items = ... whatever kinds of items being managed
 *   protected boolean[] used = new boolean[MAX_AVAILABLE];
 *
 *   protected synchronized Object getNextAvailableItem() {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (!used[i]) {
 *          used[i] = true;
 *          return items[i];
 *       }
 *     }
 *     return null; // not reached
 *   }
 *
 *   protected synchronized boolean markAsUnused(Object item) {
 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
 *       if (item == items[i]) {
 *          if (used[i]) {
 *            used[i] = false;
 *            return true;
 *          } else
 *            return false;
 *       }
 *     }
 *     return false;
 *   }
 * }}</pre>
 *
 * <p>Before obtaining an item each thread must acquire a permit from
 * the semaphore, guaranteeing that an item is available for use. When
 * the thread has finished with the item it is returned back to the
 * pool and a permit is returned to the semaphore, allowing another
 * thread to acquire that item.  Note that no synchronization lock is
 * held when {@link #acquire} is called as that would prevent an item
 * from being returned to the pool.  The semaphore encapsulates the
 * synchronization needed to restrict access to the pool, separately
 * from any synchronization needed to maintain the consistency of the
 * pool itself.
 *
 * <p>A semaphore initialized to one, and which is used such that it
 * only has at most one permit available, can serve as a mutual
 * exclusion lock.  This is more commonly known as a <em>binary
 * semaphore</em>, because it only has two states: one permit
 * available, or zero permits available.  When used in this way, the
 * binary semaphore has the property (unlike many {@link java.util.concurrent.locks.Lock}
 * implementations), that the &quot;lock&quot; can be released by a
 * thread other than the owner (as semaphores have no notion of
 * ownership).  This can be useful in some specialized contexts, such
 * as deadlock recovery.
 *
 * <p> The constructor for this class optionally accepts a
 * <em>fairness</em> parameter. When set false, this class makes no
 * guarantees about the order in which threads acquire permits. In
 * particular, <em>barging</em> is permitted, that is, a thread
 * invoking {@link #acquire} can be allocated a permit ahead of a
 * thread that has been waiting - logically the new thread places itself at
 * the head of the queue of waiting threads. When fairness is set true, the
 * semaphore guarantees that threads invoking any of the {@link
 * #acquire() acquire} methods are selected to obtain permits in the order in
 * which their invocation of those methods was processed
 * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
 * applies to specific internal points of execution within these
 * methods.  So, it is possible for one thread to invoke
 * {@code acquire} before another, but reach the ordering point after
 * the other, and similarly upon return from the method.
 * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
 * honor the fairness setting, but will take any permits that are
 * available.
 *
 * <p>Generally, semaphores used to control resource access should be
 * initialized as fair, to ensure that no thread is starved out from
 * accessing a resource. When using semaphores for other kinds of
 * synchronization control, the throughput advantages of non-fair
 * ordering often outweigh fairness considerations.
 *
 * <p>This class also provides convenience methods to {@link
 * #acquire(int) acquire} and {@link #release(int) release} multiple
 * permits at a time.  Beware of the increased risk of indefinite
 * postponement when these methods are used without fairness set true.
 *
 * <p>Memory consistency effects: Actions in a thread prior to calling
 * a "release" method such as {@code release()}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions following a successful "acquire" method such as {@code acquire()}
 * in another thread.
 *
 * @author Doug Lea
 * @since 1.5
 */
public class Semaphore implements java.io.Serializable {

  private static final long serialVersionUID = -3222578661600680210L;
  /**
   * All mechanics via AbstractQueuedSynchronizer subclass
   */
  private final Sync sync;

  /**
   * Synchronization implementation for semaphore.  Uses AQS state
   * to represent permits. Subclassed into fair and nonfair
   * versions.
   */
  abstract static class Sync extends AbstractQueuedSynchronizer {

    private static final long serialVersionUID = 1192457210091910933L;

    Sync(int permits) {
      setState(permits);
    }

    final int getPermits() {
      return getState();
    }

    final int nonfairTryAcquireShared(int acquires) {
      for (; ; ) {
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
            compareAndSetState(available, remaining)) {
          return remaining;
        }
      }
    }

    protected final boolean tryReleaseShared(int releases) {
      for (; ; ) {
        int current = getState();
        int next = current + releases;
        if (next < current) // overflow
        {
          throw new Error("Maximum permit count exceeded");
        }
        if (compareAndSetState(current, next)) {
          return true;
        }
      }
    }

    final void reducePermits(int reductions) {
      for (; ; ) {
        int current = getState();
        int next = current - reductions;
        if (next > current) // underflow
        {
          throw new Error("Permit count underflow");
        }
        if (compareAndSetState(current, next)) {
          return;
        }
      }
    }

    final int drainPermits() {
      for (; ; ) {
        int current = getState();
        if (current == 0 || compareAndSetState(current, 0)) {
          return current;
        }
      }
    }
  }

  /**
   * NonFair version
   */
  static final class NonfairSync extends Sync {

    private static final long serialVersionUID = -2694183684443567898L;

    NonfairSync(int permits) {
      super(permits);
    }

    protected int tryAcquireShared(int acquires) {
      return nonfairTryAcquireShared(acquires);
    }
  }

  /**
   * Fair version
   */
  static final class FairSync extends Sync {

    private static final long serialVersionUID = 2014338818796000944L;

    FairSync(int permits) {
      super(permits);
    }

    protected int tryAcquireShared(int acquires) {
      for (; ; ) {
        if (hasQueuedPredecessors()) {
          return -1;
        }
        int available = getState();
        int remaining = available - acquires;
        if (remaining < 0 ||
            compareAndSetState(available, remaining)) {
          return remaining;
        }
      }
    }
  }

  /**
   * Creates a {@code Semaphore} with the given number of
   * permits and nonfair fairness setting.
   *
   * @param permits the initial number of permits available. This value may be negative, in which
   * case releases must occur before any acquires will be granted.
   */
  public Semaphore(int permits) {
    sync = new NonfairSync(permits);
  }

  /**
   * Creates a {@code Semaphore} with the given number of
   * permits and the given fairness setting.
   *
   * @param permits the initial number of permits available. This value may be negative, in which
   * case releases must occur before any acquires will be granted.
   * @param fair {@code true} if this semaphore will guarantee first-in first-out granting of
   * permits under contention, else {@code false}
   */
  public Semaphore(int permits, boolean fair) {
    sync = fair ? new FairSync(permits) : new NonfairSync(permits);
  }

  /**
   * Acquires a permit from this semaphore, blocking until one is
   * available, or the thread is {@linkplain Thread#interrupt interrupted}.
   *
   * <p>Acquires a permit, if one is available and returns immediately,
   * reducing the number of available permits by one.
   *
   * <p>If no permit is available then the current thread becomes
   * disabled for thread scheduling purposes and lies dormant until
   * one of two things happens:
   * <ul>
   * <li>Some other thread invokes the {@link #release} method for this
   * semaphore and the current thread is next to be assigned a permit; or
   * <li>Some other thread {@linkplain Thread#interrupt interrupts}
   * the current thread.
   * </ul>
   *
   * <p>If the current thread:
   * <ul>
   * <li>has its interrupted status set on entry to this method; or
   * <li>is {@linkplain Thread#interrupt interrupted} while waiting
   * for a permit,
   * </ul>
   * then {@link InterruptedException} is thrown and the current thread's
   * interrupted status is cleared.
   *
   * @throws InterruptedException if the current thread is interrupted
   */
  public void acquire() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
  }

  /**
   * Acquires a permit from this semaphore, blocking until one is
   * available.
   *
   * <p>Acquires a permit, if one is available and returns immediately,
   * reducing the number of available permits by one.
   *
   * <p>If no permit is available then the current thread becomes
   * disabled for thread scheduling purposes and lies dormant until
   * some other thread invokes the {@link #release} method for this
   * semaphore and the current thread is next to be assigned a permit.
   *
   * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
   * while waiting for a permit then it will continue to wait, but the
   * time at which the thread is assigned a permit may change compared to
   * the time it would have received the permit had no interruption
   * occurred.  When the thread does return from this method its interrupt
   * status will be set.
   */
  public void acquireUninterruptibly() {
    sync.acquireShared(1);
  }

  /**
   * Acquires a permit from this semaphore, only if one is available at the
   * time of invocation.
   *
   * <p>Acquires a permit, if one is available and returns immediately,
   * with the value {@code true},
   * reducing the number of available permits by one.
   *
   * <p>If no permit is available then this method will return
   * immediately with the value {@code false}.
   *
   * <p>Even when this semaphore has been set to use a
   * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
   * immediately acquire a permit if one is available, whether or not
   * other threads are currently waiting.
   * This &quot;barging&quot; behavior can be useful in certain
   * circumstances, even though it breaks fairness. If you want to honor
   * the fairness setting, then use
   * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
   * which is almost equivalent (it also detects interruption).
   *
   * @return {@code true} if a permit was acquired and {@code false} otherwise
   */
  public boolean tryAcquire() {
    return sync.nonfairTryAcquireShared(1) >= 0;
  }

  /**
   * Acquires a permit from this semaphore, if one becomes available
   * within the given waiting time and the current thread has not
   * been {@linkplain Thread#interrupt interrupted}.
   *
   * <p>Acquires a permit, if one is available and returns immediately,
   * with the value {@code true},
   * reducing the number of available permits by one.
   *
   * <p>If no permit is available then the current thread becomes
   * disabled for thread scheduling purposes and lies dormant until
   * one of three things happens:
   * <ul>
   * <li>Some other thread invokes the {@link #release} method for this
   * semaphore and the current thread is next to be assigned a permit; or
   * <li>Some other thread {@linkplain Thread#interrupt interrupts}
   * the current thread; or
   * <li>The specified waiting time elapses.
   * </ul>
   *
   * <p>If a permit is acquired then the value {@code true} is returned.
   *
   * <p>If the current thread:
   * <ul>
   * <li>has its interrupted status set on entry to this method; or
   * <li>is {@linkplain Thread#interrupt interrupted} while waiting
   * to acquire a permit,
   * </ul>
   * then {@link InterruptedException} is thrown and the current thread's
   * interrupted status is cleared.
   *
   * <p>If the specified waiting time elapses then the value {@code false}
   * is returned.  If the time is less than or equal to zero, the method
   * will not wait at all.
   *
   * @param timeout the maximum time to wait for a permit
   * @param unit the time unit of the {@code timeout} argument
   * @return {@code true} if a permit was acquired and {@code false} if the waiting time elapsed
   * before a permit was acquired
   * @throws InterruptedException if the current thread is interrupted
   */
  public boolean tryAcquire(long timeout, TimeUnit unit)
      throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
  }

  /**
   * Releases a permit, returning it to the semaphore.
   *
   * <p>Releases a permit, increasing the number of available permits by
   * one.  If any threads are trying to acquire a permit, then one is
   * selected and given the permit that was just released.  That thread
   * is (re)enabled for thread scheduling purposes.
   *
   * <p>There is no requirement that a thread that releases a permit must
   * have acquired that permit by calling {@link #acquire}.
   * Correct usage of a semaphore is established by programming convention
   * in the application.
   */
  public void release() {
    sync.releaseShared(1);
  }

  /**
   * Acquires the given number of permits from this semaphore,
   * blocking until all are available,
   * or the thread is {@linkplain Thread#interrupt interrupted}.
   *
   * <p>Acquires the given number of permits, if they are available,
   * and returns immediately, reducing the number of available permits
   * by the given amount.
   *
   * <p>If insufficient permits are available then the current thread becomes
   * disabled for thread scheduling purposes and lies dormant until
   * one of two things happens:
   * <ul>
   * <li>Some other thread invokes one of the {@link #release() release}
   * methods for this semaphore, the current thread is next to be assigned
   * permits and the number of available permits satisfies this request; or
   * <li>Some other thread {@linkplain Thread#interrupt interrupts}
   * the current thread.
   * </ul>
   *
   * <p>If the current thread:
   * <ul>
   * <li>has its interrupted status set on entry to this method; or
   * <li>is {@linkplain Thread#interrupt interrupted} while waiting
   * for a permit,
   * </ul>
   * then {@link InterruptedException} is thrown and the current thread's
   * interrupted status is cleared.
   * Any permits that were to be assigned to this thread are instead
   * assigned to other threads trying to acquire permits, as if
   * permits had been made available by a call to {@link #release()}.
   *
   * @param permits the number of permits to acquire
   * @throws InterruptedException if the current thread is interrupted
   * @throws IllegalArgumentException if {@code permits} is negative
   */
  public void acquire(int permits) throws InterruptedException {
    if (permits < 0) {
      throw new IllegalArgumentException();
    }
    sync.acquireSharedInterruptibly(permits);
  }

  /**
   * Acquires the given number of permits from this semaphore,
   * blocking until all are available.
   *
   * <p>Acquires the given number of permits, if they are available,
   * and returns immediately, reducing the number of available permits
   * by the given amount.
   *
   * <p>If insufficient permits are available then the current thread becomes
   * disabled for thread scheduling purposes and lies dormant until
   * some other thread invokes one of the {@link #release() release}
   * methods for this semaphore, the current thread is next to be assigned
   * permits and the number of available permits satisfies this request.
   *
   * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
   * while waiting for permits then it will continue to wait and its
   * position in the queue is not affected.  When the thread does return
   * from this method its interrupt status will be set.
   *
   * @param permits the number of permits to acquire
   * @throws IllegalArgumentException if {@code permits} is negative
   */
  public void acquireUninterruptibly(int permits) {
    if (permits < 0) {
      throw new IllegalArgumentException();
    }
    sync.acquireShared(permits);
  }

  /**
   * Acquires the given number of permits from this semaphore, only
   * if all are available at the time of invocation.
   *
   * <p>Acquires the given number of permits, if they are available, and
   * returns immediately, with the value {@code true},
   * reducing the number of available permits by the given amount.
   *
   * <p>If insufficient permits are available then this method will return
   * immediately with the value {@code false} and the number of available
   * permits is unchanged.
   *
   * <p>Even when this semaphore has been set to use a fair ordering
   * policy, a call to {@code tryAcquire} <em>will</em>
   * immediately acquire a permit if one is available, whether or
   * not other threads are currently waiting.  This
   * &quot;barging&quot; behavior can be useful in certain
   * circumstances, even though it breaks fairness. If you want to
   * honor the fairness setting, then use {@link #tryAcquire(int,
   * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
   * which is almost equivalent (it also detects interruption).
   *
   * @param permits the number of permits to acquire
   * @return {@code true} if the permits were acquired and {@code false} otherwise
   * @throws IllegalArgumentException if {@code permits} is negative
   */
  public boolean tryAcquire(int permits) {
    if (permits < 0) {
      throw new IllegalArgumentException();
    }
    return sync.nonfairTryAcquireShared(permits) >= 0;
  }

  /**
   * Acquires the given number of permits from this semaphore, if all
   * become available within the given waiting time and the current
   * thread has not been {@linkplain Thread#interrupt interrupted}.
   *
   * <p>Acquires the given number of permits, if they are available and
   * returns immediately, with the value {@code true},
   * reducing the number of available permits by the given amount.
   *
   * <p>If insufficient permits are available then
   * the current thread becomes disabled for thread scheduling
   * purposes and lies dormant until one of three things happens:
   * <ul>
   * <li>Some other thread invokes one of the {@link #release() release}
   * methods for this semaphore, the current thread is next to be assigned
   * permits and the number of available permits satisfies this request; or
   * <li>Some other thread {@linkplain Thread#interrupt interrupts}
   * the current thread; or
   * <li>The specified waiting time elapses.
   * </ul>
   *
   * <p>If the permits are acquired then the value {@code true} is returned.
   *
   * <p>If the current thread:
   * <ul>
   * <li>has its interrupted status set on entry to this method; or
   * <li>is {@linkplain Thread#interrupt interrupted} while waiting
   * to acquire the permits,
   * </ul>
   * then {@link InterruptedException} is thrown and the current thread's
   * interrupted status is cleared.
   * Any permits that were to be assigned to this thread, are instead
   * assigned to other threads trying to acquire permits, as if
   * the permits had been made available by a call to {@link #release()}.
   *
   * <p>If the specified waiting time elapses then the value {@code false}
   * is returned.  If the time is less than or equal to zero, the method
   * will not wait at all.  Any permits that were to be assigned to this
   * thread, are instead assigned to other threads trying to acquire
   * permits, as if the permits had been made available by a call to
   * {@link #release()}.
   *
   * @param permits the number of permits to acquire
   * @param timeout the maximum time to wait for the permits
   * @param unit the time unit of the {@code timeout} argument
   * @return {@code true} if all permits were acquired and {@code false} if the waiting time elapsed
   * before all permits were acquired
   * @throws InterruptedException if the current thread is interrupted
   * @throws IllegalArgumentException if {@code permits} is negative
   */
  public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
      throws InterruptedException {
    if (permits < 0) {
      throw new IllegalArgumentException();
    }
    return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
  }

  /**
   * Releases the given number of permits, returning them to the semaphore.
   *
   * <p>Releases the given number of permits, increasing the number of
   * available permits by that amount.
   * If any threads are trying to acquire permits, then one
   * is selected and given the permits that were just released.
   * If the number of available permits satisfies that thread's request
   * then that thread is (re)enabled for thread scheduling purposes;
   * otherwise the thread will wait until sufficient permits are available.
   * If there are still permits available
   * after this thread's request has been satisfied, then those permits
   * are assigned in turn to other threads trying to acquire permits.
   *
   * <p>There is no requirement that a thread that releases a permit must
   * have acquired that permit by calling {@link Semaphore#acquire acquire}.
   * Correct usage of a semaphore is established by programming convention
   * in the application.
   *
   * @param permits the number of permits to release
   * @throws IllegalArgumentException if {@code permits} is negative
   */
  public void release(int permits) {
    if (permits < 0) {
      throw new IllegalArgumentException();
    }
    sync.releaseShared(permits);
  }

  /**
   * Returns the current number of permits available in this semaphore.
   *
   * <p>This method is typically used for debugging and testing purposes.
   *
   * @return the number of permits available in this semaphore
   */
  public int availablePermits() {
    return sync.getPermits();
  }

  /**
   * Acquires and returns all permits that are immediately available.
   *
   * @return the number of permits acquired
   */
  public int drainPermits() {
    return sync.drainPermits();
  }

  /**
   * Shrinks the number of available permits by the indicated
   * reduction. This method can be useful in subclasses that use
   * semaphores to track resources that become unavailable. This
   * method differs from {@code acquire} in that it does not block
   * waiting for permits to become available.
   *
   * @param reduction the number of permits to remove
   * @throws IllegalArgumentException if {@code reduction} is negative
   */
  protected void reducePermits(int reduction) {
    if (reduction < 0) {
      throw new IllegalArgumentException();
    }
    sync.reducePermits(reduction);
  }

  /**
   * Returns {@code true} if this semaphore has fairness set true.
   *
   * @return {@code true} if this semaphore has fairness set true
   */
  public boolean isFair() {
    return sync instanceof FairSync;
  }

  /**
   * Queries whether any threads are waiting to acquire. Note that
   * because cancellations may occur at any time, a {@code true}
   * return does not guarantee that any other thread will ever
   * acquire.  This method is designed primarily for use in
   * monitoring of the system state.
   *
   * @return {@code true} if there may be other threads waiting to acquire the lock
   */
  public final boolean hasQueuedThreads() {
    return sync.hasQueuedThreads();
  }

  /**
   * Returns an estimate of the number of threads waiting to acquire.
   * The value is only an estimate because the number of threads may
   * change dynamically while this method traverses internal data
   * structures.  This method is designed for use in monitoring of the
   * system state, not for synchronization control.
   *
   * @return the estimated number of threads waiting for this lock
   */
  public final int getQueueLength() {
    return sync.getQueueLength();
  }

  /**
   * Returns a collection containing threads that may be waiting to acquire.
   * Because the actual set of threads may change dynamically while
   * constructing this result, the returned collection is only a best-effort
   * estimate.  The elements of the returned collection are in no particular
   * order.  This method is designed to facilitate construction of
   * subclasses that provide more extensive monitoring facilities.
   *
   * @return the collection of threads
   */
  protected Collection<Thread> getQueuedThreads() {
    return sync.getQueuedThreads();
  }

  /**
   * Returns a string identifying this semaphore, as well as its state.
   * The state, in brackets, includes the String {@code "Permits ="}
   * followed by the number of permits.
   *
   * @return a string identifying this semaphore, as well as its state
   */
  public String toString() {
    return super.toString() + "[Permits = " + sync.getPermits() + "]";
  }
}
